1. Field of the Invention
This invention relates to the thickening of muds having clay-sized particles and yield pseudo-plastic properties produced by industrial processes and, more particularly, although not exclusively, to the thickening of red muds produced during the production of alumina from bauxite.
2. Discussion of the Prior Art
The most economical way of obtaining alumina from bauxite ore involves the Bayer process. This utilizes the reactions of solid aluminum hydroxide (Al(OH).sub.3) and aluminum oxide trihydrate and monohydrate (Al.sub.2 O.sub.3.3H.sub.2 O and Al.sub.2 O.sub.3.H.sub.2 O) with aqueous caustic soda to form soluble sodium aluminate. The soluble compound is separated from insoluble impurities, precipitated as alumina trihydrate and calcined to form commercial grades of alumina.
The insoluble impurities of the ore remaining after the caustic soda treatment are known as "red muds" and these are thoroughly washed to remove soluble products before being discarded. The washing is normally carried out in a counter-current manner in several stages. Each stage involves mixing the mud slurry underflow from an earlier stage with the clarified overflow from a later stage. After each washing step, a flocculant is added to the mixture and the resulting mud flocs are allowed to settle and thicken in a clarifier/thickener vessel. The thickened mud is drawn off from the bottom of the vessel and passed to the next stage and the clarified overflow is injected as wash liquor in the previous stage. The washing normally takes place in a feedwell or feed pipe before the mixture is discharged into the thickener/clarifier. It is usual to stack several shallow thickener/clarifier vessels in a vertical manner to save space and operating costs. Nevertheless, the capital costs and space requirements of such a washing and thickening procedure are substantial because of the large surface area requirements of such thickener/clarifier vessels. One way of reducing such capital costs would be to enhance the ability of each thickener/clarifier to thicken the mud because the efficiency of the washing procedure is increased as the solids content of the mud from each stage is increased. The mud removed as underflow from a conventional thickener/clarifier has a solids content of 15 to 30%. The use of synthetic flocculants instead of the traditional starch has recently improved this figure to 17-33%, but this improvement is not sufficient to alter significantly the economics of the washing procedure.
The use of deep thickeners (as discussed by Dr. J. L. Chandler in an article entitled "Development of the Deep Thickener" produced by the Technical Development Department of Alcan Jamaica Company in January 1980) instead of conventional shallow thickener/clarifiers of large surface area improves the thickening effect, but a deposit of inactive (i.e. thick and immobile) mud builds up at the periphery of the vessel. This build up of inactive mud is undesirable because it reduces the active volume of the vessel available for settling and thickening and because the inactive mud tends to slump downwards periodically and block the mud outlet, or it may build up towards the level of the liquid surface, thus impairing the clarity of the overflow. Attempts to use conventional raking mechanisms to clear the entire inner volume of the vessel to prevent inactive mud build up have proven costly and impractical in view of the high torque requirements of the conventional mechanisms when used in this way.
Moreover, while deep thickeners are capable of increasing the solids content of the mud outflow, the natural thickening rate of the mud flocs is fairly slow and improvements could be obtained if this rate could be increased.